1. Field of the Invention
This invention relates to a process for the production of expanded particles of a polymeric material.
2. The Prior Art
It is known to produce a foamed article of a polymeric material by first forming pre-expanded particles of the polymeric material and, then, heating the pre-expanded particles within a mold to further expand the pre-expanded particles to a desired shape. Such pre-expanded particles may be prepared by, for example, a method disclosed in Japanese Examined Patent Publication No. 56-1344 and including the steps of: (a) providing expandable particles of a polymeric material containing a volatile organic blowing agent, (b) dispersing the particles in water contained in a closed vessel, (c) heating the dispersion to a temperature higher than the softening point of the polymeric material while maintaining the pressure within the vessel at a pressure above the vapor pressure of the blowing agent, and (d) discharging the dispersion from the vessel for subjecting same to an atmosphere maintained at a pressure lower than that within the vessel, thereby the particles discharged are expanded. Examples of the volatile blowing agent include propane, butane, heptane, hexane, cyclobutane, cyclopentane, trichlorofluoromethane and dichlorodifluoromethane.
Such a method of preparing pre-expanded particles, however, has been found to involve some problems due to the use of such volatile blowing agents. Firstly, the temperature at which the pre-expansion is performed should be limited to a specific, narrow range, since otherwise the expandable particles are swelled with the blowing agents which are good solvents for the polymeric material. Therefore, the expansion ratio which is dependent upon the expansion temperature is unavoidably limited within a narrow range. Secondly, some of these blowing agents are dangerous because of their toxicity and inflammability and require the replacement with air after completion of the pre-expansion. Such a replacement requires the recovery of the blowing agents not only because of their expensiveness but also because of their tendency to destroy the ozone layer surrounding the earth.